SC3-RAV is a comprehensive triaxial downhole seismic testing (DST) data reduction, analysis, and display software package. The user is provided with an extensive set of mathematical tools and patented (U.S. Patent #5,177,709 / Canadian Patent # 2,077,387) algorithms in deriving DST (e.g., seismic cone (SC)) interval velocities and attenuation Q values and quantifying vertical seismic profiling. In addition, SC3-RAV has functionality for reviewing frequency spectrums, comparing unfiltered and digitally filtered traces and plotting interval velocity profiles. SC3-RAV also provides extensive chart editing, plotting, and exporting functionality. SC3-RAV includes the following features:
The Seismic Analysis menu option allows the user to process acquired seismic traces so that important Geotechnical design parameters are obtained. The Seismic Analysis option contains the submenus of Interval Velocities, Attenuation Analysis, Polarization Analysis, Data Interpolation, Depth Profile, Data Stack, Data Interpolation, Polarity Change and Signal Decay.
The Crosscorrelation Technique implements a patented (U.S. Patent #5,177,709 / Canadian Patent # 2,077,387) mathematical algorithm in deriving DST interval velocity profiles . This technique derives DST interval velocities based upon cross-correlating the wavelets recorded at consecutive depth increments (Baziw 1993). The value of the time shift at the maximum cross-correlation value is assumed to be the relative travel time difference for the wavelet to travel the depth increment. This technique has the following advantages over the standard Reverse Polarity Technique
The Batch Job Analysis option facilitates the user to process many seismic traces with similar filter parameters in a batch mode. In addition, Batch Job Analysis allows for the estimation of the arrival times for each depth increment and the implementation of linear least squares regression (LLSR) in deriving interval velocities. The LLSR technique facilitates in the minimization of the variability of the interval velocities. The LLSR utilizes three adjacent crosscorrelation relative arrival times, the corresponding time series depths of acquisition and a reference arrival time and depth to determine the slope of the best fitting line through the three points. The slope of the best fit line is defined to be the interval velocity.
The Forward Modeling / Downhill Simplex Method (FMDSM) utilizes seismic ray tracing and optimal estimation techniques in deriving DST interval velocities. The standard techniques implemented in DST interval velocities rely upon obtaining reference P and S wave arrival times as the seismic receiver is advanced into the ground. By assuming a straight ray travel path from source to seismic receiver and calculating relative reference arrival time differences, interval DST velocities are obtained.
The FMDSM offers distinct advantages over conventional DST velocity profile estimation methods. Some of the advantages over conventional techniques provided by the FMDSM are outlined as follows:
In the FMDSM the user is provided with user friendly interfaces for specifying seismic wave arrival times and crosscorrelation time shifts information (derived from Depth Profile and Crosscorrelation Technqiue) with corresponding weights within a Windows compatible database.
Once the necessary data has been inputted, the user executes the FMDSM. Upon completion of the FMDSM, SC3-RAV displays the estimated interval velocities graphically with the option to Ray Trace.
The FMDSM derived interval velocities are also stored in the previously described database so that they can be utilized in other applications, report generation or for future reference.
SC3-RAV allows for extensive polarization analysis. Seismic sources are often designed to generate either dominantly P and SV waves or dominantly SH waves due to the fundamentally different behaviour of P, SV and SH waves at a boundary. When a P or SV wave strikes a boundary four outgoing waves are generated: SV and P, reflected and transmitted. In contrast, a SH wave will only generate reflected and transmitted SH waves, thus simplifying the recorded seismic time series.
The figure below illustrates the source wavelets (P, SV, and SH) impacting upon a triaxial seismic cone array. As it is shown below, the P wave’s particle motion is in the same direction as the ray path, the SH wavelet’s particle motion is perpendicular to the ray path and is parallel to the horizontal ground surface and the SV wavelet’s particle motion is also perpendicular to the ray path but along the vertical normal to the ray path. The symbols φ and θyx define the ray path’s angle of incidences in spherical coordinates,where 0
# θyx # 2π and 0 # φ #π.
The calculation of the incident angles of the particle motion of the source seismic wavelet allows the investigator to derive the full (three dimensional) seismic source waveform response. This information provides insight into the validity of straight ray propagation and into the tilt of the borehole or SC rods. In addition, incident angle information allows for the derivation of soil properties which require full waveform information (e.g., attenuation and dynamic compaction analysis). SC3-RAV calculates an independent velocity estimate for the full source seismic waveform.
If the primary source wavelet is a P-wave (ie., particle motion in same directions as ray path) the three-component time series, X(t), Y(t), and Z(t) can be rotated into the local ray path coordinate system with one longitudinal component in the compression wave (P) direction and two transverse components in the shear wave (SV and SH) directions. This axis rotation simplifies analysis of the different source wavelets (i.e., P, SV, and SH) where the investigator can subsequently determine interval velocities for the P, SV and SH waves. In addition, when utilizing SC3-RAV’s analysis techniques it is advantageous to firstly rotate the source wave responses on the X and Y axes onto the full waveform axis if only a SH source wavelet is present. This significantly simplifies post analysis, because one is analyzing one full waveform response as opposed to component responses on the X and Y axes.
The energy dissipation (in the form of heat) of a wavelet as it travels through a medium is referred to as its attenuation or absorption by the medium. In general terms, the in-situ stratigraphy acts as both a low pass filter and an attenuator as a seismic wavelet travels through it. The decrease in amplitude of the wavelet due to absorption is exponential and can be defined in both the distance and time domains. SC3-RAV allows for attenuation analysis from acquired seismic time series data by utilizing the spectral ratio technique. The spectral ratio algorithm carries out the following tasks:
The figure below illustrates the final output from the Interval Analysis. The values illustrated at the top of the figure are estimates of the Quality Factor (Q), spectral slope, cross correlation coefficient (ξ), relative arrival time (ΔT) and average depth increment, respectively. The cross correlation coefficient is provided so that the user can quantify the accuracy of the results. A correlation coefficient value near 1.0 indicates very high correlation between traces and subsequent spectral ratio estimates are very accurate assuming that the dominant frequencies have been properly isolated. Values near zero indicate no correlation between traces and thus no weight should be given to spectral ratio and Q estimates.
SC3-RAV's Depth Profiling software option consists of two advanced vertical seismic profiling graphical interfaces which allow the user to filter and plot the captured triaxial seismic traces on a depth vs time plot, specify or automate trend lines for preliminary velocity estimation and display peak particle accelerations, velocities, or displacements.
The Data Stack option allows the user to post-stack acquired seismic time series data.
This analysis feature allows the investigator to increase the resolution of the captured seismic data for post processing purposes. This facilitates greater data resolution when carrying out analyses such as obtaining relative time shifts utilizing the cross-correlation function, reverse polarity analysis, spectral ratio analysis, polarization analysis, and specifying trend lines in depth profiling. In addition, SC3-RAV’s Data Interpolation software feature allows the investigator to synchronize seismic time series data captured with differing sampling rates (i.e., set data to same sampling rate).
Polarity Change
The Polarity Change option facilitates the investigator in selecting a set of
seismic data files and subsequently changing the polarity of the time series.
This option has proven very helpful when deriving interval velocities from data
which has been rotated onto the full waveform axis by utilising Polarization
Analysis.
Signal Decay
The Signal Decay option allows the investigator to minimize the effect of
possible source wavelet multiples on the first arriving source wavelet. The
technique implemented relies upon the application of an exponential decay
applied to the selected time series data after a user specified time.
The View menu option consists of the sub-menus Interval Velocities and Seismic Data. These software features allow the user to display derived seismic interval velocities (ie., Interval Velocities) and analyze seismic data on a trace by trace basis (Seismic Data).
The Utilities menu option consists of sub-menus Default GUI Settings, Sensor Type, Enable Time Delay, Shear Modulus Calculation, and Export IVF to Column Format. These software features allow the user to specify general interface settings, the type of seismic data recorded (i.e., accelerometer or geophone), enable the ability to implement time delay corrections within Depth Profiling and calculate and store Gmax values.